Limitations of Current CATV Trunk Architecture
The traditional cable television (CATV) distribution trunk is coaxial cable-based It contains a series of amplifiers placed approximately every 1/2 mile along the signal path. These amplifiers are needed to maintain the proper operational signal level At selected locations, the signals are tapped and further amplified to bring them to the customer premises.
In a typical CATV trunk there are 30-50 (some trunks contain as many as 60) amplifiers in a cascade. This large number of amplifiers brings about many unwanted problems, such as:
A failure in any of the cascaded amplifiers will cause the shutdown of the entire trunk.
The signal quality degrades significantly as the number of amplifiers increase In some existing trunks, the number of amplifiers is so large (60 or more) that it makes it impossible to add any more subscribers to the system.
Many older CATV trunks have frequency response limited to 300 MHz at most, and suffer from spurious radiation problems. Rebuilding to 450-550 MHz capability and curing the radiation problems can be a very costly effort, requiring replacement of the amplifiers and even the coaxial plant itself.
Role of Supertrunks in CATV Video Signal Distribution
Headends themselves are frequently connected with other remote video sources such as satellite-received signals or with other headends.
Linking headends with satellite signals or with other headends has traditionally been done with supertrunks. Because of the very high quality of the transmission required, the supertrunk is typically FDM-FM (Frequency Division Multiplexed--Frequency Modulated) based distribution system. Present day state of the art supertrunks can carry up to 16 channels per supertrunk.
Early supertrunks used coaxial cable as the transmission media. Recently, fiber optic-based supertrunks have been used to avoid the problems associated with the conventional coaxial cable-based supertrunks.
Fiber-based supertrunks not only deliver video transparently (with no system-added degradation), but they also offer the system designed with totally new system capability--the fiber offers very wide bandwidth capability, low losses, very small weight and size, no RFI (Radio Frequency Interference) or EMI (ElectroMagnetic Interference) and rapidly dropping cost. Currently, 16 channels using FDM-FM can be transmitted over 40 km of fiber with no repeaters and with very high performance - SNR (Signal to Noise Ratio) of 65 db or better. FDM-FM supertrunks have excellent video performance but have limitations, especially with non-standard video signals. If scrambled signals are transmitted over the supertrunk using FM, the demodulated baseband video will not have the sync or other suitable clamp reference signals, and this makes it virtually impossible for the following AM (Amplitude Modulation) modulator to operate properly. Thus, in the traditional CATV architecture, the supertrunk terminates in hub, where the signal is first FM demodulated and than descrambled to recover the proper sync and other missing reference signals. Following the descrambling operation, the video signal is VSB (Vestigial Side Band)-AM modulated for transmission over a CATV distribution trunk. This results in bulky multirack equipment configuration and very high cost per channel.